An electric power steering apparatus (EPS) which provides a steering system of a vehicle with a steering assist torque (an assist torque) by a rotational force of a motor, applies the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears by using a driving force of the motor which is controlled by electric power supplied from an electric power supplying section. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs a feed-back control of a motor current. The feed-back control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a handle (a steering wheel) 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a pinion-and-rack mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the steering shaft 2 is provided with a steering angle sensor 14 for detecting a steering angle θ and a torque sensor 10 for detecting a steering torque Th of the handle 1, and a motor 20 for assisting the steering torque of the handle 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command (a steering assist command) on the basis of the steering torque Th detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor by means of a voltage control command value Vref obtained by performing a compensation or the like for the calculated current command value. A steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle (a motor rotational angle) θ from a rotational position sensor such as a resolver which is connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vs from the CAN 40. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
In such an electric power steering apparatus, the control unit 30 mainly comprises a central processing unit (CPU) (including a micro processor unit (MPU) and a micro controller unit (MCU)), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque Th from the torque sensor 10 and the vehicle speed Vs from the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Th and the vehicle speed Vs using an assist map or the like. The calculated current command value Iref1 is added with a compensation signal CM for improving characteristics from a compensating section 34 at an adding section 32A. The current command value Iref2 after addition is limited of the maximum value thereof at a current limiting section 33. The current command value Irefm limited of the maximum value is inputted into a subtracting section 32B, whereat a detected motor current value Im is subtracted from the current command value Irefm.
The subtracted result ΔI (=Irefm−Im) at the subtracting section 32B is current-controlled such as a proportional-integral (PI) at a PI-control section 35. The voltage control value Vref obtained by the current control and a modulation signal (a triangle wave carrier) CF are inputted into a PWM-control section 36, whereat a duty thereof is calculated. The motor 20 is PWM-driven by an inverter 37 with a PWM signal calculated the duty. The motor current value Im of the motor 20 is detected by a motor current detection means 38 and is inputted into the subtracting section 32B for the feed-back.
The compensating section 34 adds a self-aligning torque (SAT) detected or estimated and an inertia compensation value 342 at an adding section 344. The added result is further added with a convergence control value 341 at an adding section 345. The added result is inputted into the adding section 32A as the compensation signal CM, thereby to improve the control characteristics.
Recently, a 3-phase brushless motor is mainly used as an actuator of the electric power steering apparatus, and since the electric power steering apparatus is on-vehicle products, the operating temperature range is wide. From a view point of a fail-safe, a dead time of the inverter to drive the motor needs greater than that for general industrial purposes that home appliances (“industrial equipment”<“EPS”). Generally, since a switching device (for example, a field-effect transistor (FET)) has a delay time when it is turned-OFF, a direct current link is shorted when the switching devices of an upper-arm and a lower-arm are simultaneously turned-ON or turned-OFF. In order to prevent the above problem, a time (a dead time) that the switching devices of both arms are turned-OFF, is set.
As a result, a current waveform is distorted, and a responsibility of the current control and a steering feeling go down. For example, in a state that the handle is near on-center of the steering, a discontinuous steering feeling and the like due to the torque ripple are occurred. Since the back-EMF (electromotive force) voltage of the motor in a while speed steering or a high speed steering, and the interference voltage between the windings operate as the disturbance against the current control, a steering follow-up performance and the steering feeling in the turn-back steering are badly affected.
A q-axis to control the torque being the coordinate axis of a rotor of the 3-phase brushless motor and a d-axis to control the magnetic field strength are independently set and has a relation that the d-axis and the q-axis is 90°. Thus, a vector control system to control the currents (the d-axis current command value and the q-axis current command value) corresponding to respective axes with the vector, is known.
FIG. 3 shows a configuration example in a case that a 3-phase brushless motor 100 is driving-controlled by the vector control system. A d-axis current command value id* and a q-axis current command value iq*, which are calculated at the current command value calculating section (not shown) based on the steering torque Th, the vehicle speed Vs and so on, of a dq-axes coordinate system of two axes are respectively inputted into subtracting sections 131d and 131q, and current deviations Δid* and Δiq* obtained in the subtracting sections 131d and 131q are respectively inputted into PI-control sections 120d and 120q. Voltage command values vd and vq PI-controlled at the PI-control sections 120d and 120q are respectively inputted into a subtracting section 141d and an adding section 141q, and command voltages Δvd and Δvq obtained at the subtracting section 141d and the adding section 141q are inputted into a dq-axes/3-phase alternating current (AC) converting section 150. Voltage command values Vu*, Vv* and Vw* converted into three phases at the dq-axes/3-phase AC converting section 150 are inputted into a PWM-control section 160, and the motor 100 is driven with PWM-signals UPWM, VPWM, WPWM based on calculated 3-phase duty command values (Dutyu, Dutyv, Dutyw) via the inverter (inverter-applying voltage VR) 161 constituted by a bridge configuration of an upper-arm and a lower-arm as shown in FIG. 4. The upper-arm comprises FETs Q1, Q3, Q5 and the lower-arm comprises FETs Q2, Q4, Q6 serving as switching devices.
3-phase motor currents iu, iv and iw of the motor 100 are detected by current detectors 162, and the detected 3-phase motor currents iu, id and iw are inputted into the 3-phase AC/dq-axes converting section 130. The 2-phase feed-back current id and iq that are converted at the 3-phase AC/dq-axes converting section 130 are respectively subtraction-inputted into subtracting sections 131d and 131q, and further inputted into a d-q non-interference control section 140. 2-phase voltages vd1* and vq1* from the d-q non-interference control section 140 are respectively inputted into the subtracting section 141d and the adding section 141q, and the command voltages Δvd and Δvq are calculated at the subtracting section 141d and the adding section 141q. The command voltages Δvd and Δvq are inputted into the dq-axes/3-phase AC converting section 150, and the motor 100 is driven via the PWM-control section 160 and the inverter 161.
Further, a rotational sensor or the like is attached to the motor 100, and a motor rotational angle θ and a motor rotational number (velocity) ω are outputted from an angle detecting section 110 to process the sensor signal. The motor rotational angle θ is inputted into the dq-axes/3-phase AC converting section 150 and the 3-phase AC/dq-axes converting section 130, and the motor rotational number ω is inputted into the d-q non-interference control section 140.
The electric power steering apparatus of the vector control system described above is an apparatus to assist a steering of a driver, and also a sound and a vibration of the motor, a torque ripple and the like are transmitted to the driver as a force sense via the steering wheel. The FETs are generally used as power devices to drive the inverter, and the current is applied to the motor. In a case that the 3-phase motor is used, FETs, which are connected in series for respective phases, of the upper-arm and the lower-arm are used as shown in FIG. 4. Although the FETs of the upper-arm and the lower-arm are alternatively turned-ON and turned-OFF, the FET does not simultaneously turn-ON and turn-OFF in accordance with a gate signal since the FET is not an ideal switching device. Therefore, a turn-ON time and a turn-OFF time are needed. Consequently, if an ON-command for the upper-arm FET and an OFF-command for the lower-arm FET are simultaneously inputted, there is a problem that the upper-arm FET and the lower-arm FET simultaneously turn-ON and the upper-arm and the lower-arm become short circuits. There is a difference between the turn-ON time and the turn-OFF time of the FET. Thus, when the command is inputted into the FETs at the same time, the FET immediately turns-ON in a case that the turn-ON time is short (for example, 100 [ns]) by inputting the ON-command to the upper-FET, and reversely, the FET does not immediately turn-OFF in a case that the turn-OFF time is long (for example, 400 [ns]) by inputting the OFF-command to the lower-FET. In this way, a state (for example, between 400 [ns]−100 [ns], ON-ON) that the upper-FET is “ON” and the lower FET is “ON”, often momentarily occurs.
In this connection, in order that the upper-arm FET and the lower-arm FET do not simultaneously turn-ON, the ON-signal is usually given to the gate driving circuit with a predetermined period being a dead time. Since the dead time is nonlinear, the current waveform is distorted, the responsibility of the control is badly affected and the sound, the vibration and the torque ripple are generated. In a column type electric power steering apparatus, since an arrangement of the motor directly connected to a gear box which is connected by the handle and the column shaft made of steel is extremely near the driver in the mechanism, it is necessary to especially consider the sound, the vibration, the torque ripple and the like due to the motor in comparison with a downstream type electric power steering apparatus.
Conventionally, as a method to compensate the dead time of the inverter, there are methods to add the compensation value to the dead time by detecting a timing occurring the dead time and to compensate the dead time by using a disturbance observer on the dq-axes in the current control.
The electric power steering apparatus to compensate the dead time is disclosed in, for example, Japanese Patent No. 4681453 B2 (Patent Document 1) and Japanese Unexamined Patent Publication No. 2015-171251 A (Patent Document 2). In Patent Document 1, there is provided a dead band compensation circuit that generates a model current based on the current command values by inputting the current command values into a reference model circuit of the current control loop including the motor and the inverter, and compensates the influence of the dead time of the inverter based on the model current. Further, in Patent Document 2, there is provided a dead time compensating section to correct based on the dead time compensation value for the duty command value, and the dead time compensating section comprises a basic compensation value calculating section to calculate a basic compensation value being a basic value of the dead time compensation value based on the current command value and a filtering section to perform a filtering-process corresponding to a low pass filter (LPF) for the basic compensation value.